This application is based upon and claims benefit of priority of Japanese Patent Applications No. 2001-45258 filed on Feb. 21, 2001 and No. 2001-72610 filed on Mar. 14, 2001, the contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to a fuel evaporation control system for use in an automobile vehicle, and more particularly to a device for detecting deterioration of a canister used in the fuel evaporation control system.
2. Description of Related Art
A fuel evaporation control system in which gaseous fuel evaporated from a fuel tank of an automobile vehicle is absorbed by a canister and the absorbed fuel is purged into an intake pipe of an engine is known hitherto. The canister is composed of activated charcoal for absorbing the evaporated fuel and a filter for removing dusts contained in the atmospheric air. The canister is connected to a fuel tank, the intake pipe and the atmosphere through respective passages. An amount of fuel purged into the intake pipe is controlled by a control valve installed between the canister and the intake pipe.
JP-A-6-506514 and JP-A-4-265457 disclose such a fuel evaporation control system, and more particularly a technique for detecting an increase in a flow resistance in the canister. In the system disclosed in JP-A-6-506514, a negative pressure is introduced in the fuel evaporation control system, and then a purge control valve is closed. Under this situation, an atmospheric pressure and an in-tank pressure are measured. Then, a difference between both pressures is compared with a predetermined criterion value. If the pressure difference exceeds the criterion value for a predetermined period of time, it is determined that the flow resistance through the canister is abnormally increased. In other words, it is determined that the canister is deteriorated.
However, in the technique disclosed, the criterion value for determining the canister deterioration is set based on various parameters such as engine speed, an engine load, a duty ratio of the purge control valve. Accordingly, a complex process is required in setting the criterion value. Also, it is difficult to precisely determine the canister deterioration because an amount of fuel vaporizing from the fuel tank differs depending on driving conditions of the engine. Further, in the systems disclosed in both JP-A-6-506514 and JP-A-4-265457, deterioration of fuel-absorbing ability of the canister cannot be detected.
The present invention has been made in view of the above-mentioned problems, and an object of the present invention is to provide an improved fuel evaporation control system in which deterioration in air permeability and fuel-absorbing ability of the canister is accurately detected with high accuracy using a simple structure and a process.
Air-fuel ratio in a mixture gas supplied to an internal combustion engine is electronically controlled by an electronic control unit. A fuel evaporation control system is connected to such an engine control system to purge evaporated fuel from a fuel tank into the engine. The fuel evaporation control system includes a fuel tank, a canister and a control valve for controlling an amount of purge gas purged into the engine. The canister is composed of activated charcoal and a filter for removing foreign particles contained in atmospheric air. Fuel evaporated from the fuel tank is absorbed by activated charcoal in the canister, and the absorbed fuel is purged into the engine by negative pressure in an intake pipe of the engine. The amount of negative pressure in the fuel evaporation control system, which is represented by an in-tank fuel pressure, is measured by a pressure sensor.
Air-permeability in the canister decreases due to deterioration of the canister. The in-tank pressure decreases as the air-permeability in the canister decreases since a sufficient amount of air is not taken in through the canister in this case. In other words, an amount of negative pressure in the fuel evaporation control system increases according to decrease of the air-permeability in the canister. The amount of such pressure decrease becomes larger as an amount of purge flow fed to the engine becomes larger. The in-tank pressure is measured under a low purge flow and a high purge flow, and a pressure difference between two in-tank pressures measured in such a manner is compared with a predetermined criterion. If the pressure difference exceeds the criterion, it is determined that the air-permeability in the canister is abnormally low.
Since the in-tank pressure is measured under two different amounts of purge flow, and the pressure difference is compared with a predetermined criterion, the air-permeability decrease, or the flow resistance increase, in the canister is accurately detected in a simple process. The in-tank pressure may be measured after a predetermined period of time has lapsed after the amount of purge flow reaches a predetermined level, a low level or a high level, in order to obtain stabilized in-tank pressure. The detection of the air-permeability may be prohibited when the system is not normally operating or a purge gas density is too high to avoid misjudgment of the air-permeability in the canister. When the abnormally low air-permeability is detected, air-fuel ratio control system may be switched to a fail-safe mode in which an abrupt change in the air-fuel ratio is avoided.
In the air-fuel ratio control, an amount of fuel injected from injectors is adjusted according to an amount of fuel purged from the fuel evaporation control system in order to control the air-fuel ratio at a desired level. In this process, a purge gas density is calculated under a learning procedure. Fuel-absorbing ability of the canister is detected based on the calculated, or learned, purge gas density. The purge gas density decreases in a short period of time during the purging process if the fuel-absorbing ability becomes low due to deterioration of the canister. The highest level of purge gas density at a given purging process is calculated and memorized. When a predetermined period of time has lapsed after the purge gas density reached the highest level, the level of the purge gas density at that time is compared with the highest level. If a difference between the highest level and the present level exceeds a predetermined criterion, it is determined that the fuel-absorbing ability of the canister is abnormally low. Thus, abnormality in the fuel-absorbing ability is accurately detected based on the purge gas density. The predetermined period of time may be calculated by integrating the purge gas density in the purging process.
The air-permeability in the canister is also detected based on the purge gas density. The purge gas density at a low amount of purge gas flow is compared with the purge gas density at a high amount of purge flow. If the difference exceeds a predetermined amount, it is determined that the air-permeability of the canister is abnormally low. The air-permeability decrease due to deterioration of the canister is accurately detected in a simple manner.
According to the present invention, deterioration of the canister both in its air-permeability and in its fuel-absorbing ability is accurately detected-in a simple manner.